42 Mr. Hopkins on the Structure of the 



tion in this particular instance ; but in those localities where we observe a want of 

 this perfect accordance, we must necessarily recognise the operation of some modi- 

 fying cause. The transverse fissures just mentioned present an instance of the 

 kind ; and the modifying cause was probably the jointed structure of the mass in 

 the locality in which they exist ; for it appeared to me that the predominant joints 

 in the neighbourhood were such as would be likely to produce exactly the kind of 

 modification recognised in the directions of the transverse dislocations. 



And here we may remark, that the great tendency of an approximately uniform 

 elevatory force, in a tract of which the length is much greater than the breadth, 

 would be to produce longitudinal fissures, since the directions of greatest tension 

 would manifestly be perpendicular to the longer axis of the district, except at its 

 extremities. The formation of transverse fissures might, in such cases, be due, in 

 a considerable degree, to local irregularities in the action of the elevatory force, or 

 in the constitution of the mass. The longitudinal fissures would be formed first, 

 and their prior existence would, in a great measure, determine the directions of 

 the transverse fissures, whether the latter were formed at the next instant during 

 the same elevatory movement, or at any subsequent movement. This cause, how- 

 ever, as respects its influence in fixing the directions of the transverse fissures, 

 would frequently be less determinate than the general action of the elevatory force, 

 in fixing those of the longitudinal fissures ; and, therefore, while the latter might 

 remain sensibly unaffected, the former might be materially modified by the irregu- 

 larities above mentioned in the action of the force, or in the constitution of the 

 mass. Such modifying causes must necessarily exist in all cases ; but we can have 

 no means of calculating their efiects. In some instances they might be sufficient 

 to destroy all symmetry and apparent law in the resulting phsenomena ; but in the 

 instance before us, the accordance between observed and calculated results assures 

 us that the influence of merely local causes has been extremely small. 



In this harmony between facts and theory we observe something more than a 

 first approximation to accuracy. If nothing more had been established by observa- 

 tion than that there were certain lines of elevation of which the general directions 

 approximated to east and west, and others which were directed nearly to the north 

 and south, and a theory had been offered assigning some probable cause for this 

 law of parallelism, such theory might have been deserving of attention as a first 

 and rough approximation to a more accurate one. But the theory which I have been 

 applying to the Wealden district does not stop at this point. It not only assigns a 

 physical cause for the general law of parallelism, but, wherever the data of the 

 problem are sufficiently determinate, it shows the manner in which the lines of 

 elevation will deviate from this parallelism in each particular district. In the case 

 before us, the deviations of the longitudinal lines from rectilinearity and from 



